1. Field of the Invention
The present invention relates to a common-mode choking coil, and specifically relates to a common-mode choking coil used, for example, for an EMI eliminating filter.
2. Description of the Prior Art
FIG. 11 is a plan view showing one example of a conventional common-mode choking coil which is the background of the present invention. A common-mode choking coil 1 comprises a closed magnetic path core 2. The closed magnetic path core 2 is formed by combining two U-shaped cores 3. These two cores 3 are fixed, for example, by a spring 4 having a U-shaped cross section.
A bobbin 5 is mounted on one leg of the closed magnetic path core 2. Two wire-wound parts 5b are formed by collars 5a on the bobbin 5. On the two wire-wound parts 5b, coils 6a and 6b are formed respectively by winding a conductor wire. The coils 6a and 6b are formed by winding the conductor wire in respective directions opposite to each other. When these coils 6a and 6b are formed, to wind the conductor wire a required number of turns, the conductor wire is wound from one end to the other end of the wire-wound part 5b of the bobbin 5, and thereafter the conductor wire is wound from this end toward the original end in a superposed manner on the previously wound wire. Thus, the coils 6a and 6b are formed by winding the conductor wire in a multi-layered fashion. The ends of the windings of the coils 6a and 6b are connected to terminals 7 formed, for example, on the bobbin 5.
However, such a common-mode choking coil has a large distributed capacitance because it is formed by winding the conductor wire in a multi-layered fashion. For this reason, the self resonance frequency of the coil is lowered, and a large insertion loss characteristic cannot be obtained in the high frequency region, and thus there has been the problem that an effective elimination of noise cannot be performed.
To solve such a problem, it has been considered to form a single-layer-wound coil by winding the conductor wire in only one layer. The distributed capacitance of the coil can be reduced by winding the coil in a single layer. However, when the coil is formed by single-layer winding, the conductor wire has to be wound by a large number of turns in one layer to obtain a predetermined inductance value, and a large coil length is required. When the coil length becomes large, the magnetic path length of the core also becomes long as a result and the magnetic resistance becomes large, and eventually the insertion loss characteristic is reduced.
Also, in the conventional common-mode choking coil, two U-shaped cores are fixed together in the combined state, and therefore a part such as a spring is required. For this reason, the manufacturing cost of the whole common-mode choking coil is raised due to the cost of this part. Furthermore, the required work for connecting the ends of the coil to the terminals also cause a rise in the cost of the common-mode choking coil.